Update PackageResolverTest.cs

Author: kasperk81

src/libraries/Microsoft.Extensions.DependencyModel/tests/PackageResolverTest.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.IO;
 using FluentAssertions;